Nitric oxide (NO) is a potent mediator produced by many mammalian cells. NO undergoes numerous reactions producing several redox forms. NO can act as a cellular signaling molecule, as a damaging species, or as an antioxidant. This depends on concentration, duration of exposure, and on which targets with which it interacts. Among cellular targets, modification of cellular thiols has been shown to be an important signaling mechanism for NO redox forms. This may lead to S-nitrosoglutathione formation or to protein S-nitrosylation. These modifications of cellular thiols can be considered an "on" signal. This proposal focuses on reverse NO-mediated thiol modification as a potential "off" signal. Preliminary experiments have found that NO modification of protein thiols leads to formation of protein mixed disulfides, which are known to be effectively reduced by the combined activities of glutaredoxin and the GSH redox cycle. The applicant proposes that this mechanism is the off signal. As such, the GSH redox cycle may play a protective role through the same mechanism. The goals of the proposal are to investigate mechanisms for interaction between NO, cellular thiols and the GHS redox cycle using endothelial cells exposed to exogenous NO donors or cell models in which NO is produced endogenously. The overall hypothesis is that the GSH redox cycle regulates NO mediated signaling by interacting directly with NO related species and by reversing protein thiol modification mechanisms involving protein disulfide formation. It is assumed that similar mechanisms protect cells from the damaging effects of NO.